1. Field of the Invention
The present invention relates to a method of magneto-optically recording information on a magneto-optical recording medium with an intensity-modulated laser beam.
2. Description of the Prior Art
There have been proposed magneto-optical recording methods capable of overwriting information on multilayer magneto-optical recording mediums with an intensity-modulated laser beam so that newly recorded information replaces the previously recorded information (see, for example, Japanese laid-open patent publications Nos. 62-175948 and 63-52355).
As shown in FIG. 1 of the accompanying drawings, a multilayer magneto-optical recording medium 1, such as a magneto-optical disk, for magneto-optically recording information thereon comprises a transparent substrate 2 such as a glass substrate, a resin substrate, or the like, a dielectric film 3 disposed on one surface of the transparent substrate 2, a recording and reproducing layer 4 and a recording assistance layer 5 with perpendicular magnetic anisotropy which are successively deposited on the dielectric film 3 by way of continuous sputtering or the like, and a protective film 6 such as a nonmagnetic metallic film, a dielectric film, or the like coated on the surface of the recording assistance layer 5.
The recording and reproducing layer 4 may be made of a rare-earth-rich (RE-rich) material such as TbFeCo, for example, which has a large coercive force H.sub.C1 at normal temperature T.sub.R (ranging from -20.degree. to 60.degree. C.) and a low Curie temperature T.sub.C1. The recording assistance layer 5 may be made of a transition-metal-rich (TM-rich) material such as GdTbFeCo, for example, which has a small coercive force H.sub.C2 at normal temperature T.sub.R and a high Curie temperature T.sub.C2.
Information is recorded on the magneto-optical recording medium 1 as follows: As shown in FIG. 2 of the accompanying drawings, the recording assistance layer 5 is magnetized in an upward direction as indicated by the arrows, and the recording and reproducing layer 4 is magnetized in upward and downward directions as indicated by the arrows. The layers 4, 5 which are magnetized in the upward direction are combined in a first state ST.sub.A, and the layers 4, 5 which are magnetized in the downward and upward directions, respectively, are combined in a second state ST.sub.B. The first and second states ST.sub.A, ST.sub.B serve to represent the recorded information "0" and the recorded information "1", respectively.
A process of recording the information "0" on the magneto-optical recording medium 1 which is in either the first state ST.sub.A or the second state ST.sub.B will be described below. First, a laser beam having a first laser power P.sub.L which is sufficient to achieve a temperature T.sub.ER that is higher than the Curie temperature T.sub.C1 of the recording and reproducing layer 4 and lower than the Curie temperature T.sub.C2 of the recording assistance layer 5, is applied to the magneto-optical recording medium 1 for a predetermined period of time to bring the recording and reproducing layer 4 into a third state ST.sub.C in which the direction of magnetization is unstable.
Thereafter, the magneto-optical recording medium 1 is cooled on its own accord thereby to enable the exchange force acting in the boundary between the layers 4, 5 to orient the direction of magnetization of the recording and reproducing layer 4 in the same upward direction as the recording assistance layer 5. In this manner, the magneto-optical recording medium 1 is brought from the first state ST.sub.A representing "0" or the second state ST.sub.B representing 1"` to the first state ST.sub.A representing "0".
To record the information "1" on the magneto-optical recording medium 1 which is in either the first state ST.sub.A or the second state ST.sub.B, the magneto-optical recording medium 1 is irradiated for a predetermined period of time with a laser beam having a second laser power P.sub.H, higher than the first laser power P.sub.L, which is sufficient to achieve a temperature T.sub.REC that is close to the Curie temperature T.sub.C2 of the recording assistance layer 5. At the same time, a downward recording magnetic field H.sub.REC ranging from 300 to 500 Oe, for example, higher than the coercive force H.sub.CR of the recording assistance layer 5 at the temperature T.sub.REC is applied to the magneto-optical recording medium 1.
Now, the magneto-optical recording medium 1 is brought into a fourth state ST.sub.D in which the direction of magnetization of the recording and reproducing layer 4 is unstable and the recording assistance layer 5 is magnetized downwardly. Thereafter, the magneto-optical recording medium 1 is cooled on its own accord, enabling the exchange force acting in the boundary between the layers 4, 5 to orient the direction of magnetization of the recording and reproducing layer 4 in the same downward direction as the recording assistance layer 5. Such a magnetized condition of the layers 4, 5 is referred to as a fifth state ST.sub.E.
Thereafter, an upward initializing magnetic field H.sub.ini ranging from 3 to 5 kOe which is smaller than the coercive force H.sub.C1 of the recording and reproducing layer 4 at the normal temperature T.sub.R and larger than the coercive force H.sub.C2 of the recording assistance layer 5 at the normal temperature T.sub.R is applied to the magneto-optical recording medium 1 to orient the direction of magnetization of only the recording assistance layer 5 upwardly. In this manner, the magneto-optical recording medium 1 is brought from the first state ST.sub.A representing "0" or the second state ST.sub.B representing "1" to the second state ST.sub.B representing "1".
According to the above magneto-optical recording process, the direction of magnetization of the layers 4, 5 can be controlled into an upward or downward direction by controlling the intensity of the laser power. Therefore, using a single laser beam, new information can be overwritten directly on the magneto-optical recording medium 1 to replace the old recorded information, without any separate process of erasing the old recorded information.
In the above magneto-optical recording process, the magneto-optical recording medium 1 is always irradiated with either the laser beam of the first low-level laser power P.sub.L to record "0" or the laser beam of the second high-level laser power P.sub.H to record "1" while erasing the previously recorded information. Therefore, the temperature distribution of the magneto-optical recording medium 1 which is irradiated with pulses of the laser beam tends to be in a broader range than it is according to a general magneto-optical recording process in which only the laser beam of the second laser power P.sub.H is switched on and off.
More specifically, it is assumed that after the laser beam of the second laser power P.sub.H is applied to the magneto-optical recording medium 1 to record the information "1", the laser beam of the first laser power P.sub.L is applied to record erase the recorded information, i.e., to record the information "0". The laser beam of the first laser power P.sub.L is applied before a region around the pit in the magneto-optical recording medium 1 whose temperature has been increased by the laser beam of the second laser power P.sub.H is sufficiently cooled down. At this time, due to the thermal interference between the laser beams, the recording and reproducing layer 4 tends to be heated to a temperature that exceeds the Curie temperature T.sub.C1 at which the direction of magnetization of the recording and reproducing layer 4 is unstable, and that is close to the Curie temperature T.sub.C2 of the recording assistance layer 5.
When information is overwritten according to the conventional magneto-optical recording process, pits formed in the recording and reproducing layer 4 is liable to be irregular in configuration, resulting in a poor C/N ratio, i.e., higher noise. If two adjacent pits are closely spaced from each other, it is difficult to recognize the pits separately, and hence the error rate is increased.
The region of the magneto-optical recording medium 1 which is irradiated with the laser beam of the first laser power P.sub.L is thermally affected to a different degree depending on the interval at which the laser beam of the second laser power P.sub.H is applied. Since the region of the magneto-optical recording medium 1 is thermally affected to a different degree, an optimum range of the first laser power P.sub.L varies with the result that the range (margin) of the first laser power P.sub.L which is optimum for various laser pulse intervals becomes narrow.
While it is necessary to set the low-level laser power P.sub.L accurately to a constant level, it is actually difficult to select such a constant level and set the laser power to the selected constant level. The narrow margin for the low-level laser power is responsible for an increase in the BER (Byte Error Rate).
In an attempt to eliminate the above drawbacks, there has been proposed a magneto-optical recording process as disclosed in Japanese laid-open patent publication No. 2-244443. According to the disclosed magneto-optical recording process, desired information is magneto-optically recorded on a magneto-optical recording medium with an intensity-modulated laser beam, and the laser beam is turned off for a predetermined period of time .tau..sub.0 following the application of the laser beam of the high-level laser power P.sub.H.